Mixotrophs are important components of the bacterioplankton, phytoplankton, microzooplankton, and (sometimes) zooplankton in coastal and oceanic waters. Bacterivory among the phytoplankton may be important for alleviating inorganic nutrient stress and may increase primary production in oligotrophic waters. Mixotrophic phytoflagellates and dinoflagellates are often dominant components of the plankton during seasonal stratification. Many of the microzooplankton grazers, including ciliates and Rhizaria, are mixotrophic owing to their retention of functional algal organelles or maintenance of algal endosymbionts. Phototrophy among the microzooplankton may increase gross growth efficiency and carbon transfer through the microzooplankton to higher trophic levels. Characteristic assemblages of mixotrophs are associated with warm, temperate, and cold seas and with stratification, fronts, and upwelling zones. Modeling has indicated that mixotrophy has a profound impact on marine planktonic ecosystems and may enhance primary production, biomass transfer to higher trophic levels, and the functioning of the biological carbon pump.

Arranging organisms into functional groups aids ecological research by grouping organisms (irrespective of phylogenetic origin) that interact with environmental factors in similar ways. Planktonic protists traditionally have been split between photoautotrophic "phytoplankton" and phagotrophic "microzooplankton". However, there is a growing recognition of the importance of mixotrophy in euphotic aquatic systems, where many protists often combine photoautotrophic and phagotrophic modes of nutrition. Such organisms do not align with the traditional dichotomy of phytoplankton and microzooplankton. To reflect this understanding, we propose a new functional grouping of planktonic protists in an eco-physiological context: (i) phagoheterotrophs lacking phototrophic capacity, (ii) photoautotrophs lacking phagotrophic capacity, (iii) constitutive mixotrophs (CMs) as phagotrophs with an inherent capacity for phototrophy, and (iv) non-constitutive mixotrophs (NCMs) that acquire their phototrophic capacity by ingesting specific (SNCM) or general non-specific (GNCM) prey. For the first time, we incorporate these functional groups within a foodweb structure and show, using model outputs, that there is scope for significant changes in trophic dynamics depending on the protist functional type description. Accordingly, to better reflect the role of mixotrophy, we recommend that as important tools for explanatory and predictive research, aquatic food-web and biogeochemical models need to redefine the protist groups within their frameworks.

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Ten years of pH measurements (1990 to 1999) in the surface waters of the eutrophic Mariager Fjord, Denmark, revealed profound seasonal variation. Typically, pH was relatively constant around 8 from January to March, increased during spring, reached maximum levels in July to August (9 to 9.7), and declined during autumn to about 8 in October. The influence of pH on the growth rate of phytoplankton was tested on 3 species (Ceratium lineatum, Heterocapsa triquetra and Prorocentrum minimum) in laboratory experiments. The growth rate was highest at pH 7.5 to 8.0 in all species. The growth rate of C. lineatum declined by ~20% at pH 8.3 to 8.5, while a similar reduction in the growth rate in H. triquetra and P. minimum was observed at pH 8.8 to 8.9. C. lineatum stopped growing above pH 8.8, while growth ceased at about pH 9.45 in H. triquetra and 9.6 in P. minimum. Compilation of literature data on pH and phytoplankton growth suggested that while some species cannot grow at pH 8.4, others are able to grow up to pH 10. However, none of the species studied can attain their maximum growth rate above pH 9. Competition experiments using a mixture of C. lineatum, H. triquetra and P. minimum always resulted in the species with the highest pH tolerance (P. minimum) outcompeting the other species, irrespective of the initial pH value. The role of high pH in the succession of marine phytoplankton in nature is discussed.

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